Matrix acidizing is the most commonly used stimulation method for carbonate reservoirs. Many studies have already been done to predict acidization patterns. However, most of these studies focus on chemical reaction kinetics. In this study, core was acidized using conditions similar to field situations. The acidization pattern was then analyzed using X-ray computed tomography. The results indicate that the usual acidization pattern with 15% HC1 is the formation of a single wormhole from each perforation. This acidization pattern is due to rock heterogeneity, limited acid diffusion and viscous fingering of the acid.
Conventional X-ray radiography has a long and successful history both in industrial and medical applications, of displaying the image of various objects (e.g., chest X-ray). The more recent technology is X-ray computed tomography (CT) - also known as computer assisted (axial) tomography (CAT). The Greek word "tomo" refers to a plane or slice.
An important characteristics of CT is that the plane of interest alone is displayed, whereas conventional X-ray radiography superimposes all layers on a single sheet of film, including those overlaying and under laying the plane of interest (Figure 1). The result of CT is, therefore, an image of outstanding detail and resolution.
X-ray CT was pioneered by G. N. Hounsfield of EMI Ltd. for medical use, the first equipment being installed in 1972. To date, this is still the area where the technology has made its most significant impact. Yet, it has also gained acceptance in other fields. In the petroleum industry, its use was pioneered by Vinegar and Wellington of Shell Development Co. The technique has been successfully used in core analyses l-6 and fluid flow studies 6–8. Most of the fluid flow studies which have been conducted are in conventional oil recovery investigations. However, Sedgwick, et. al. used the technique to study heavy oil flow behavior 8. Other examples of industrial applications reported in the literature include materials failure analyses and measurement of solids concentrations in water 9,10.
Matrix acidizing of carbonate reservoirs involves injecting acid into a formation at below formation fracture pressure. Acid dissolves portions of the rock, the result being increased permeability. The most commonly used acid is 15% HC1. The typical Chemical reactions are:
CaCO3 + 2HCl → CaCl2 + H2O + CO2 (Limestone)
CaMg(CO3)2 + 4HC1 → CaC12 + MgC12 + 2H2O + 2CO2 (Dolomite)
In addition to chemical reactions, physical parameters are important to create well interconnected acid-etched channels. These parameters include the rock's permeability, porosity and pore geometry. Acid parameters such as flow rate, viscosity and reactivity can also have impact on the treatment.
Many studies have been undertaken to investigate acidization patterns using reservoir cores. Conventionally, an acidized core is set in resin and cut into slices for observation under a microscope. A replica of the acid-etched channels can be made by filling the channels with resins or low melting-point alloys and dissolving the surrounding rock materials when the cast is set.
